GaN is one of Group III-V compound semiconductors, and has a wurtzite-type crystal structure belonging to a hexagonal system.
A GaN substrate is a substrate configured by only a GaN crystal. A C-plane GaN substrate is commercially produced, and is mainly used as a substrate for InGaN-based light-emitting devices (laser diodes and light-emitting diodes).
On the other hand, a non-polar or semi-polar GaN substrate attracts attention as a new substrate for nitride semiconductor devices including a light-emitting device (Non-Patent Document 1).
Non-polar GaN substrates which particularly attract attention include an M-plane substrate, namely, a (10-10) substrate. Semi-polar GaN substrates which particularly attract attention include a (20-21) substrate, a (20-2-1) substrate, a (30-31) substrate and a (30-3-1) substrate.
The name of crystal plane or Miller index, attached to the name of a GaN substrate, is that of a low index plane parallel to or most nearly parallel to the front surface of the substrate. The front surface refers to a surface intended to be used for formation of a semiconductor device or epitaxial growth of a crystal, of two main surfaces of the substrate. Another main surface which is not the front surface is referred to as the back surface.
Therefore, a GaN substrate called an “M-plane substrate” or “(10-10) substrate” is a GaN substrate whose low index plane parallel to or most nearly parallel to the front surface thereof is an M-plane, namely, {10-10}. A crystal plane where the absolute values of all the integers h, k, m and l of the Miller indices {hkml} are 3 or less is usually a low index plane.
A non-polar or semi-polar GaN substrate can be produced by a method where a bulk GaN crystal grown on a C-plane GaN template in the c-axis direction by use of an HVPE method is sliced parallel to a desired non-polar or semi-polar plane.
The non-polar or semi-polar GaN substrate produced by this method, however, has an elongated shape and its size in the direction of the orthogonal projection of the c-axis on its main surface is in the order of millimeters. The reason is that the thickness of a bulk GaN crystal with low dislocation density, which can be stably grown on a C-plane GaN template by an HVPE method, is in the order of millimeters. This method cannot provide a large-area substrate like a 2-inch substrate (disk-shaped substrate having a diameter of 2 inches).
In order to solve the above problem, a tiling method is proposed. In the tiling method, a GaN crystal is grown on an aggregate seed. The aggregate seed is configured by closely arranging a plurality of GaN substrates (tile seeds) having the same plane orientation, on a planar surface, and one example thereof is illustrated in FIG. 1.
With reference to FIG. 1, four tile seeds 10 are arranged on a flat surface to configure one aggregate seed S10. An HVPE method can be used to grow a GaN crystal 20 on a main surface of the aggregate seed S10 in a normal direction of the main surface, as illustrated in FIG. 2. That is, a GaN crystal 20 collectively covering the plurality of tile seeds 10 can be grown (Patent Document 1 and 2).
The GaN crystal grown on the aggregate seed is processed into a disk-shaped GaN substrate. Alternatively, a seed substrate is again produced from the GaN crystal, and a GaN crystal epitaxially grown on the seed substrate by a vapor phase method is processed into a disk-shaped GaN substrate.
As illustrated in FIG. 3, a flat surface called an “orientation flat (OF)” is provided on the outer circumference of a disk-shaped GaN substrate.
In a plain view of the substrate, the outer circumference forms a straight line where the OF is provided. The length of such portion where the outer circumference forms a straight line is called an “OF length”. The OF length is demanded to be less than 20 mm for a substrate having a nominal diameter of 2 inches (about 5 cm), less than 40 mm for a substrate having a nominal diameter of 4 inches (about 10 cm), and less than 60 mm for a substrate having a nominal diameter of 6 inches (about 15 cm).